Ongoing efforts to effect major economies in hookup to fiber-optic networks have as one of their goals the extension of fiber-optic service, with its inherent broadband capability, to ordinary subscribers. One such effort is disclosed in U.S. patent application Ser. No. 08/234,955, filed. In that application, a multiplicity of low cost lasers are embedded in respective fiber-extended cavities for tunability and are referenced by servo loops to a remote, stable laser source, whose temperature characteristics are thus conferred on each of those low cost lasers. The servos are inexpensive, yet they prevent laser output stability from being compromised by numerous thermal, mechanical and output coupling problems. As a result, major economies are achievable compared to traditional devices and architectures. However, the said application does not also disclose appropriately low cost modulation means; If ordinary modulation means were employed, their relatively high cost, on the order of several times the cost of the tunable lasers disclosed in the application would substantially diminish those economies.
To illustrate, one of the least costly wideband modulating means is an external lithium niobate waveguide modulator. In such a modulator, an input signal is split in two and transmitted along parallel paths in lithium niobate waveguides. By using a modulating signal to differentially bias the two paths, phase shifts are introduced which cause destructive interference when the two paths are later re-merged. Another type of wideband modulator is based on electro-absorption within the laser. If used with ordinary laser sources, these types of modulators may add on the order of one-fifth or less of the cost of the laser to system costs. However, if used with the previously described low cost lasers, the same modulators would add on the order of triple the laser cost to system costs. Clearly, a truly low cost modulator is needed if such low cost lasers are to be utilized advantageously.
To fully understand the special coupling and modulation requirements of such extended-cavity lasers, a brief description of their structure and operation is given.
Application Ser. No. 08/234,955 relates to a class of devices known as fiber-extended-cavity lasers. In that class of lasers, a modified laser diode is typically used as the active element. While an ordinary laser diode has two reflective ends that serve to define the resonant optical cavity that is required for lacing to take place, laser diodes used in fiber-extended cavities have one of those ends anti-reflectively coated so they can no longer lase, and that end is coupled to an optical waveguide, typically a side-polished fiber, that is, a portion of an ordinary optical fiber from one side of which most of the cladding has been removed.
By positioning a feedback grating at the polished portion of the fiber, the grating will be coupled to the evanescent wave that is accessible there. The grating serves as the second reflective end needed to complete a resonant cavity.
In a preferred embodiment of the invention disclosed in U.S. Pat. No. 5,315,436, of which U.S. patent application Ser. No. 08/234,955 is a Continuation In Part, continuous tuning is effected by use of a divergent grating moved along a precise path. Symmetrical or asymmetrical gratings can also be used, and divergence may be from a true vertex or an effective vertex.
U.S. Pat. No. 5,315,436 also discloses two other methods of continuously tuning a semiconductor laser that use gratings with parallel lines. In all three methods, the feedback grating is loosely, grazingly coupled to the evanescent wave. Such grazing coupling ensures that the grating lines along the entire grating participate substantially equally in the feedback process, as is required for optimum sidemode suppression. In accordance with the well known and easily proved fact that such sidemode suppression requires that the grating length be at least as long as the cavity length, the gratings used in each of these three tuning methods are relatively long.
The referenced patent and patent application make use of these tunable lasers as a component of what is called an Offset Wavelength Tracker (OWT).
An OWT is a tracking spectrometer that incorporates a tunable laser, such as those just described, and that automatically tunes that laser to output a wavelength that lies at a desired offset from a reference wavelength fed to the OWT. The offset is established by the design of the OWT. The tunable laser it incorporates may be modulated by the modulating means disclosed herein.
Although OWTs serve important functions in fiber-optic network architectures, illustratively the generation of hierarchically ordered downstream and upstream wavelength arrays, they are critical to the stability of the tunable lasers themselves. They resolve a number of stability issues stemming from thermal, mechanical and coupling problems. They confer on the numerous inexpensive laser diodes at varying ambient temperatures in a communications network the temperature stability of a single, high quality, temperature controlled wavelength reference source at a central office. They eliminate the need for an isolator between such laser diodes and the optical lines they drive. They eliminate the need to keep track of downstream and upstream wavelength assignments, a major record-keeping task. They permit all the required wavelengths to be generated with a small number of OWTs of slightly different design, permitting the economies of scale associated with the extensive use of substantially the same few components. They even confer on the network substantial immunity to installer error. In any network where a reference wavelength is available, it is advantageous to embed any of the tunable lasers disclosed in the referenced patent and application in an OWT, if only to ensure their stability. Indeed, except for certain special non-network applications, the OWT is, properly speaking, the tunable source, rather than the stand-alone laser diode with its associated tuning grating.
As earlier noted, a feature present in all three of the tunable laser structures mentioned above is their long cavity length, typically on the order of an inch. Typical laser diodes have an active region that is between 100-500 microns long, so the cavity is between 50 and 250 times longer than the cavity of an ordinary Fabry-Perot laser. This presents an opportunity to directly modulate the laser while reducing the wavelength pulling that occurs with such modulation by approximately two orders of magnitude.
A central design goal for the extended cavities discussed herein is that they be as short as possible. There are two main reasons for this.
First, the feedback grating used for tuning also effects the sidemode suppression that is critical to the prevention of cross-channel interference in dense wavelength division multiplexing. For optimal sidemode suppression, the grating must be at least as long as the cavity. Secondly, as will be shown, the maximum modulation rate of the novel modulation devices and method disclosed herein is inversely proportional to cavity length.
Since the means by which the laser diode is coupled to the fiber of the optical cavity critically impacts both the length of the feedback grating (and thus its cost, since long gratings are costly) and the maximum achievable modulation rate, those coupling means will be dealt with at length.